For a long time, the traditional means of adapting the limited dynamic range of a film or an optoelectronic image sensor to the widely varying lighting conditions in a scene has been the mechanical shutter. With a mechanical shutter, one can control the effectively deposited amount of incident radiation on the photosensitive area by selecting different exposure times, during which the shutter is open, while it is closed for the rest of the time. The effective fraction of detected to incident radiation is therefore determined by the duty cycle of the pulse-width modulation of the incident light through the mechanical shutter function.
An optoelectronic rather than a mechanical shutter is described in RU-2′204′973. Said patent makes use of liquid crystal elements whose polarization state can be changed as a function of an applied voltage. The device can be switched from a state of high absorption (shutter closed) to a state of low absorption (shutter open).
An electronic shutter function that is equivalent to a mechanical shutter function can be obtained with interline-transfer charge-coupled devices (IT-CCD), as described by A. J. P. Theuwissen in “Solid-state imaging with charge-coupled devices”, Kluwer, Dordrecht, 1995. The exposure time is started with an electrical reset operation, which clears the storage site from all previously accumulated photocharges. During the exposure time, incident light is converted into photocharges, and these photocharges are accumulated. The exposure time is ended by transporting the accumulated photocharges under an optically shielded storage area. Also in the case of the IT-CCD the effective fraction of detected to incident radiation is determined by the duty cycle of the pulse-width modulation of the incident light through the electronic shutter function. For this reason, the switching action is binary (on-off), and neither an electrically controllable optical attenuator nor the demodulation/mixing with a continuously varying time-function is possible.
U.S. Pat. Nos. 6,667,768 and 6,556,244 describe alternative electronic shutters in image sensors that can be purely realized with industry-standard complementary metal-oxide-semiconductor (CMOS) technology. A proportional number of photocharges as a function of incident radiation energy is generated in photodiodes, and these photocharges are stored in capacitors, under the control of transistors that either electrically connect the capacitors to the photodiodes, or disconnect the electrical path between capacitors and photodiodes. In this way, an electronic shutter function is realized. Also for these inventions the switching action is binary (on-off), and neither an electrically controllable optical attenuator nor a demodulation/mixing with a continuously varying time-function is possible.
U.S. Pat. No. 5,856,667 teaches the use of CCD photodetection, switching and storage devices for the binary demodulation/mixing with a binary time-function exhibiting low noise performance, in particular for applications in optical time-of-flight range imaging. The switching action is. binary (on-off), and neither an electrically controllable optical attenuator nor a demodulation/mixing with a continuously varying time-function is possible.
An alternative approach to optoelectronic mixing is described in WO-98/10255, according to which isolated gates are switched such that all charges photogenerated under a photosite are directed to either one of two charge storage devices, where they are accumulated for subsequent processing and readout. Also in this teaching, the switching action is binary (on-off), and neither an electrically controllable optical attenuator nor a demodulation/mixing with a continuously varying time-function is possible.
The known variable optical attenuators acting on a complete image at a time, such as the one described in U.S. Pat. No. 6,665,485, employ an additional attenuating element, usually placed between the imaging lens and the surface of the image sensor. Examples include varying optical density (gradient) filters, or liquid-crystal-based attenuator elements. None of these electrically controllable optical attenuators can be implemented monolithically in the image sensor itself.
The realization of a correlation pixel capable of carrying out the correlation of a photogenerated current signal with an arbitrary time-function is described in S. Ando and A. Kimachi, “Correlation Image Sensor: Two-Dimensional Matched Detection of Amplitude-Modulated Light”, IEEE Transactions on Electron Devices, Vol. 50, No. 10 (2003), pp. 2059-2066. The multiplication of the photocurrent with the convolution function is carried out with a multitude of transistors that are connected in series with a photodiode, and whose gates are controlled by the time-varying voltage function using different phase values. The two major disadvantages of this technique are on one hand the charge or voltage noise contributed by the channels of the multiplying transistors, and on the other hand the low operation frequency of the mixing action that is limited to a few 10 kHz, because the transistors are operated in sub-threshold where their high effective resistance leads to a low-pass RC filter action with low cutoff frequency.
WO-2004/001354 describes an optoelectronic sensing device for the local demodulation of a modulated electromagnetic wavefield. The device consists of a resistive, transparent electrode on top of an insulator layer that is produced over a semiconducting substrate whose surface is electrically kept in depletion. The electrode is connected with two or more contacts to a number of clock voltages that are operated synchronously with the frequency of the modulated wavefield. In the electrode and in the semiconducting substrate, a lateral electric field is created that separates and transports photogenerated charge pairs in the semiconductor to respective diffusions close to the contacts. The lateral electric field is acting in one direction only. By repetitively storing and accumulating photocharges in the diffusions, electrical signals are generated that are subsequently read out for the determination of local phase shift, amplitude and offset of the modulated wavefield. However, the device does not provide any electrically controllable optical attenuator.